ParcelBio launched with $13 million in seed financing led by Breyer Capital, with participation from General Catalyst, Y Combinator, Metaplanet, SurgePoint Capital, ZAKA VC, and additional investors.
The company said the financing will support development of its proprietary APEXm platform, which stands for Amplified and Prolonged EXpression mRNA, and advance its pipeline of next-generation mRNA therapeutics. Planned programs include an in vivo CAR-T therapy for autoimmune disease, along with additional oncology and encoded protein therapeutic programs.
ParcelBio is developing a new class of mRNA medicines designed to improve both protein expression and durability, which the company said are major limitations of current mRNA technologies. According to the company, many existing mRNA approaches require tradeoffs between potency, duration, and manufacturability, limiting broader applications beyond vaccines and short-acting therapies.
The company’s APEXm platform is designed to engineer RNA molecules that recruit native RNA-stabilizing machinery within cells, enabling higher and more sustained protein expression. ParcelBio said the platform maintains a linear mRNA architecture while improving biological performance and therapeutic durability.
ParcelBio’s lead program focuses on in vivo CAR-T therapies targeting pathogenic B cells associated with autoimmune diseases. The company said its approach aims to achieve durable B-cell depletion without viral delivery or ex vivo manufacturing, potentially enabling scalable off-the-shelf therapies.
Preclinical data presented by the company demonstrated higher and more durable protein expression compared to leading clinical mRNA designs, including stronger biological responses and improved target cell depletion in in vivo CAR-T models. ParcelBio plans to present additional preclinical data at the American Society of Gene & Cell Therapy Annual Meeting on May 14, 2026.
ParcelBio is headquartered in San Francisco and focuses on developing programmable RNA therapeutics across autoimmune disease, oncology, and protein replacement applications.
KEY QUOTES:
“mRNA has transformed medicine, but today’s technologies are fundamentally limited in how much protein they can produce and for how long. We engineered RNA to work with the cell’s machinery rather than against it, enabling meaningful improvements in both expression and durability that we believe are essential for true disease modification.”
David Weinberg, Ph.D., Chief Executive Officer And Co-Founder, ParcelBio
“ParcelBio’s durable, tunable expression is designed to collapse the tradeoffs that have constrained mRNA in chronic, deep-tissue disease, bringing durable, controllable immune reset within reach in refractory autoimmunity and beyond. Breyer Capital’s life sciences thesis is organized around the technologies that redefine what medicine can reach: programmable biology, controllable expression, and durable therapeutic impact. ParcelBio is built to advance all three.”
Morgan Cheatham, Partner And Head Of Healthcare And Life Sciences, Breyer Capital
“Most RNA platforms force a tradeoff between potency, durability, and manufacturability. Our approach eliminates that tradeoff, enabling both higher peak expression and longer duration within a manufacturable system, and opening the door to entirely new classes of medicines.”
Chris Carlson, Ph.D., Chief Scientific Officer And Co-Founder, ParcelBio
